Sensors for detecting target substances in liquid samples have heretofore been proposed. For blood glucose sensors, one example of such sensors, the liquid sample is blood and the target substance is glucose.
Most of the blood glucose sensors proposed so far are electrochemical blood glucose sensors. The electrochemical blood glucose sensor has an enzyme and an electron acceptor. The enzyme specifically reacts with blood glucose (i.e., enzyme-substrate reaction) to oxidize the glucose. The electron acceptor is converted from the oxidized form to the reduced form by accepting the electron generated by the oxidation. The reduced form of the electron acceptor is electrochemically oxidized for instance on the electrode. The concentrations of glucose in the blood, i.e., blood glucose levels are detected based on the magnitude of a current generated by this oxidation.
In a typical sensor manufacturing process, an enzyme and an electron acceptor are applied onto a substrate by applying a reagent solution in which they are dissolved. A reagent layer forms as a result of the drying of the reagent solution. The enzyme-substrate reaction proceeds as the reagent layer dissolves into a liquid sample.
Known reagent compositions used for such a reagent solution include reagent compositions containing an oxidoreductase such as glucose dehydrogenase, an electron acceptor such as a quinone compound, and an additive such as histidine and/or imidazole (see, e.g., Patent Literatures 1, 2 and 4). Reagent compositions containing an oxidoreductase such as pyrroloquinoline quinone (PQQ)-dependent glucose dehydrogenase, and a transition metal complex having a pyridyl imidazole ligand are also known (see, e.g., Patent Literature 3). Moreover, the redox potential of a quinone compound is known to be shifted to negative potentials at a particular pH in a solution containing the quinone compound and imidazole (see, e.g., Non-Patent Literature 1).